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Methyl Orange (Helianthin), SO3Na.C0H4N-:N.C0H4N(CH3)3
10 grms. sulphanilic acid.
2*5   „    anhydrous sodium carbonate (in 100 c.c. water).
3-5   „    sodium nitrite (in 20 c.c. water).
6  „     cone, hydrochloric acid (in 10 c.c. water).
6  „    dimethylaniline  (in 6 c.c. cone. HC1 and 20   c.c.
The sulphanilic acicl is dissolved in the sodium carbonate
(£ mol.) solution and the sodium nitrite (i mol.) solution added.
The mixture is cooled in ice, and the solution of hydrochloric
acid (i mol.) gradually added. The solution of dimethylaniline
(i mol.) is now poured in, and the liquid made alkaline with
caustic soda. The separation of methyl orange at once begins,
and is assisted by the addition of a little common salt (20 grains).
The precipitate is filtered at the pump, and crystallised from
hot water. Yield, nearly theoretical.
S03Na.C0H4NH2 4- NaNO2 + 2HC1 = SO3Na.C0H4N2.Cl H-
NaCl + 2H2O.
S03Na.QH4N2.Cl + CflH6N(CH3)3HCl ==
SO3H.CCH4.N2.C6H4N(CH3)2 + NaCl + HCL
S03H.C0H4N:N.C0H4N(CH3)2 + NaOH =
SO3Na.C6H4N:N.C6H4N(CH3)2 + H2O.
Properties.—Methyl orange is the sodium salt of the sul-
pl^oic acid, and dissolves in water with a yellow colour. Tlie
friracid is red, and its action as an indicator depends upon
this change on the addition of mineral acid.
Reaction.—Methyl orange is decomposed, like the majority of
azo-compounds, by stannous chloride in hydrochloric acicl Into
molecules, produced by the addition of hydrogen to tlie
Double-linked nitrogen atoms (see p. 173).
HS03.C6H4N:N.CG-H4N(CH3)0 4- 2SnCU + 4HC1 =
HS03.C6H4NHo + HoNC0H4N(CH3)2 + 2SnCl4.